Continuity mistake: After the Baxter/Rojo dinner, the background changes from nightime to daytime at the cemetery, then back to nightime again when Clint kidnaps Mirasol.
Factual error: When the "U.S." soldiers open fire on the Mexican cavalry, they employ a machine gun. The only two rapid-firing weapons in that time period were the Gatling gun, which used a magazine or drum of rounds and was fired by turning a crank, or the "Mitrailleuse" volley gun, which required at least two men to operate it, one to load the ammunition blocks, and the other to turn, once again, a crank to fire the weapon. The gun pictured has a single operator and neither a crank, nor any visible cartridge feed.
Factual error: The mitrailleuse (weapon)the bandits, who are disguised as US troops, use to gun down the Mexican soldiers is shown to have 46 barrels, yet shoots far more bullets than it can hold. Additionally, reloading is a very lengthy process.
Revealing mistake: After Joe (Clint Eastwood) shoots dead the five men in the "small house" there is a close-up shot of one of the dead men lying across the table and his eyes are still open. Joe then pushes him off the table and as his body falls the dead man closes his eyes. (01:02:38)
Visible crew/equipment: When Joe (Clint Eastwood) meets Ramón for the first time in the courtyard, they exchange a few words and then walk a short distance to a table to continue their conversation. As they walk the shadow of the boom microphone is cast on one of the pillars (the pillar with the man leaning against it and holding a rifle). (00:28:32)
Revealing mistake: When Ramón and his men massacre a detachment of Mexican soldiers by ambushing them with a Gatling gun, it's easy to tell that the soldiers are just pretending to be hit by bullets. There isn't a single bullet hole or drop of blood on any of the soldiers also in the wide shots not one horse is hit in the crossfire nor has the carriage been damaged by gunfire. (00:25:43)
Plot hole: The man with no name kills five Rojo bandits outside the city, in the so-called small house. His gunfire is heard in the town, where other Rojos respond by riding to the place of incident. In the film time, it is daytime. Shortly afterwards, the man with no name starts a fire. It is now night in the film. A fire alarm is sounded. Many scramble to extinguish it. Rojos then toss the entire town, looking for the man. Soon after that, Rojos blow up the external fence of the Baxter residence. So much ruckus in town until this point, but not a single Baxter has reacted. They still won't react until one minute later, when Rojos have set fire to their residence. 34 Baxters die on screen. Seriously, where were all these Baxters all this time? In a soundproof chamber of some sort? In coma? (01:02:10)